Solution development of xerographic latent images



hired States Patent @fhcc 3,365,324 Patented Jan. 23, E968 This invention relates to Xerography and particularly to a method or developing an electrostatic image.

In xerographic processes there are various ways of forming an electrostatic image. F or example, a photoconductive surface may be charged and then exposed to light which causes the charge to leak away in the exposed areas. The present invention is not concerned with the manner in which the electrostatic image is formed. In general, it is not concerned with the type of surface carrying the electrostatic image. The present invention is directed to the method of toning or developing this image.

Xerographic processes commonly tone the image by applying toner particles, i.e. solids, to the imagewise charged surface in the form of a powder cloud, or as a dispersion in an electrostatically inert liquid or as a mixture of toner particles and larger inert particles which are brushed or cascaded across the surface. It should be noted that even in the liquid development process, the actual toner is a solid dispersed and not dissolved in the liquid. All of these processes have in common the exposure of the image-bearing surface to an assemblage of fine, highly colored particles and this leads to a number of detrimental aspects common to all such processes.

For example, such prior toning may be considered a dirty operation. The powder cloud and carrier particle development, by their very nature, are prone to produce dust or aerosols which tend to contaminate the developing machine and the surroundings. The ink-type liquid of the dispersion processes similarly produce objectionable contamination in the development machine. One object on the present invention is to eliminate dirt completely.

Secondly, the prior processes had a serious problem in obtaining and maintaining an adequate state of subdivision of the toner particles. All such processes of development are basically grainy, being based on microscopically visible particles. The distribution of grain size in the final developed print in all prior processes bears a direct correlation to the distribution in the assemblage of toner particles used in development. The toner particles must be carefully milled and carefully screened to produce a distribution of fine particles in the first place, and then carefully handled to prevent agglomeration of the fine particles into larger ones. A second object of the present invention is to eliminate all problems of graininess and nonuniformity of particle size.

The present invention uses a clear, and in a preferred embodiment colorless, solution for toning. There is no dirt associated with it nor are there any problems involving ag lomeration of particles.

According to the present invention, development of the electrostatic image is effected by simple immersion of the image-bearing surface into, and removal from, a solution as described below or by simple swabbing of the surface with such a solution. The solution consists of a solvent which is electrically insulating even with the solute in it and of a solute, but there is no solid matter present in the solution when it is applied as a layer on the electrostatic image. Development is instantaneous with contact of the surface with the solution. No fixing is required, aside from evaporation of the solvent of the solution leaving the solute as a solid formed with image-wise distribution which may be either fringe distribution or solid area distribution as discussed further below.

The electrostatic image is usually formed on an organic photoconductor or on selenium or on Zinc oxide in resin. When the latter is used the zinc oxide in resin may be rendered hydrophilic and a hydrophobic solute can be used in the developer solution; the end result is a litho printing master as described in copending patent application Ser. No. 45,949, filed July 28, 1%0, and now U.S. Patent 3,152,969, by Donald R. Eastman. The present invention may employ any of the photoconductors used in xerography.

The phenomenon on which this method of development is based is that a latent electrostatic image when coated with an insulated liquid induces a bas-relief (thick or thin) image into this coating. When the solution is drying down, the solute forms a solid which constitutes the image. If the solute is a dye, the image is visible; if the solute is a polymer the image is a relief image which may be dyed or used for lithographic printing or reacted with a separate reagent or with the photoconductor itself to form a visible image.

Other objects and advantages of the invention will be apparent from the following description when read in connection with the accompanying drawings in which:

FIGS. 1, 2 and 3 illustrate schematically a flow chart of one process according to the invention.

FIGS. 4 and 5 similarly illustrate alternative steps in the process.

FIG. 6 illustrates a subsequent step employed in some embodiments of the invention.

in FIG. 1 a photoconductive layer 19, consisting for example of zinc oxide in resin, is coated on a conducting support 11 such as paper. The front surface of the photoconductor carries an electrostatic image indicated as negative charges 12 image-wise distributed.

According to the invention as indicated in FIG. 2, an electrically insulating solution 15 is applied to the charged surface of the layer 10. This solution is applied by dipping the sheet into the solution and removing it, or by swabbing the surface with the solution. The solution immediately forms a relief distribution having high areas 16 adjacent to the edges of the electrostatic image, i.e., adjacent to the edges of the charged areas. Practically no solution stays in the uncharged or discharged areas, although for clarity a thin layer 17 is indicated in these areas.

This fringe development, i.e. the changes in thickness of the developing liquid which occur only in areas near which there is a change in the surface charge, is similar to the fringe development well known in xerographic processes employing toner particles carried by plastic or glass beads.

In the next step of the invention illustrated by FIG. 3, the solvent of the solution is evaporated leaving the solute as a solid 20 on the electrostatically charged surface of the photoconductor it).

The solution 15 contains as its solute either colored compounds (dyes) or clear or colored polymeric compounds. All such compounds remain as solids when the solvent is evaporated. Various examples are given below. In the case of dyes, the image is immediately visible or can be further intensified. In the case of polymers these may be rendered visible by applying a suitable dye which is absorbed by the polymer and rejected by the zinc oxide or vice versa.

An alternative process employs as the clear liquid for development one which is a liquid at a temperature above room temperature, although still below the temperature at which an electrostatic image would be destroyed. Mere cooling of the liquid to room temperature then forms the solid such as 20, on the charged surface.

Also for special applications, the polymer relief image may be viewed directly as a black-and-white image using Schlieren optics which are well-known and which, by optical interference, render relief images visible as blackand-whi e. In fact, by proper selection of thicknesses in a relief image, Schlieren optics can be made to give color images.

Referring to FIG. 4, it will be noticed that the only difference between this FIG. 4 and FIG. 2 is the presence of a conducting metallic sheet 26 adjacent to the image being developed or toned. With such an arrangement, i.e. with the conductor in front of the surface during evap oration of the solvent, the solution 25 tends to congregate in and throughout each charged area as indicated in 27 rather than just on the fringes of the charged areas. Thus solid image development is obtained. The relative merits of solid images and fringe images are known in ordinary xerographic processes. As shown in FIG. 5 the evaporation of the solvent from the solution leaves a solid image 28 on the surface of the photoconductor. As mentioned previously, the present invention is not con cerned with the chemical nature of the surface on which the electrostatic images exist, except in those cases in which the formed solid 2% or 28 is reacted with the surface material itself. However, the usual forms of xerography have a photoconductor surface and hence the drawings illustrate embodiments of the invention in which the electrostatic image 12 is on a photoconductive surface 10.

Also, the image or 28 may be hydrophilic on a hydrophobic surface and used for lithoprinting or the images 26 or 28 may be hydrophobic and the photoconductor it) rendered hydrophilic by known methods. When hydrophilic solutes are used, care must be exercised to be sure that the solution is not rendered sufficiently conducting to destroy the electrostatic image, due to the conductivity induced by the water attracted thereby. In practice, this is not too difficult since the absorption of water by such solutions is slow and evaporation is rapid.

As illustrated in FIG. 6 a relief image 28 may be dyed by a dye, or reacted with a reagent, carried in a brush 29, to form a visible image. All such embodiments have the advantages of the present invention; namely the complete elimination of solids from the toning solution as it is applied to the electrostatic image. The solids are formed in situ on the surface after the clear solution has been applied.

A variety of solvents can be used. The requirements are simply that the solution shall have a specific resistance of at least 10 ohm-cm. and a dielectric constant less than 3. Suitable solvents are hydrocarbons such as hexanes, heptane, iso-octane, paint thinner, kerosene, medicinal grade mineral oil, toluene, xylene or halogenated hydrocarbons such as carbon tetrachloride, Freon 113, Freon BF, or Fluorolube FS-S.

The following examples serve to illustrate the invention although the invention is not limited to these examples.

Example 1 A xerographic latent image was prepared by imagewise exposure of a charged coating of a zinc oxide in resin photoconductor. This is a standard method of preparing an electrostatic image. The surface was swabbed with a polymer solution containing 5 g. of poly-n-butyl methacrylate in 95 g. of cyclohexane. The image was allowed to air dry. A has-relief image of solid was thus formed on the photoconductor. It was immediately visible by reflected light. It was highly contrasty when viewed through Schlieren optics.

4 Example 2 The zinc-oxide resin photoconductor of Example 1 is replaced by a photoconductor consisting of poly-N-vinyl carbazole which is known as an organic photoconductor. The same solution as used in Example 1 is applied to an electrostatic image on the surface of this organic photoconductor. When dried down, a has-relief image is formed which is visible when viewed by reflected light or when viewed through Schlieren optics, since the organic photoconductor itself is transparent.

Example 3 The electrostatic image on a Zinc oxide resin photoconductor as described in Example 1 was swabbed with a solution consisting of a 0.5% solution of A20 Oil Blue Black B (National Aniline Division of Allied Chemical) in toluene further diluted 1 to l with cyclohexane. The direct swabbing of the surface produced a color image.

Example 4 An electrostatic image on the zinc oxide resin photoconductor of Example 1 was swabbed with a 0.2% solution of diphenyl thiocarbazone diluted 1 to 1 with cyclohexane. The carbazone compound reacted with the Zinc oxide and produced a permanent visible pink image.

Example 5 An electrostatic image on the zinc oxide resin photoconductor of Example 1 was swabbed with a solution consisting of 0.5% of Sudan Black BR (General Dyestuffs) in an odorless paint thinner. The swabbing produced a color image.

Example 6 An electrostatic image on the zinc oxide resin photoconductor of Example 1 was swabbed with a solution consisting of 0.5% of Hipol Black B (Patent Chemical Co.) in a 1:3 mixture of paint thinner and toluene. The swabbing produced a color image.

Example 7 An electrostatic image on the zinc oxide resin photoconductor of Example 1 was swabbed with a solution containing 0.5% of A20 Oil Black (National Aniline) in odorless paint thinner. The swabbing produced a color image.

Having thus described various embodiments of the invention, it is pointed out that the invention is not limited thereto, but is of the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a xerographic process for the production of a visible image corresponding to an image to be recorded wherein an electrostatic charge image corresponding to said image to be recorded is produced, on an insulating surface, the method of developing said electrostatic charge image comprising the steps of applying to the entire area of said surface containing said electrostatic charge image, a layer of an electrically insulating solution having a specific resistance of 10 ohm-cm. and a dielectric constant less than 3, whereby said electrostatic charge image induces a has-relief image into said layer,

said solution being free of solid state matter and consisting of a volatile solvent and a solute that forms a solid when said solvent evaporates, and evaporating said solvent to leave said solute as a solid that is distributed on said surface in accordance with said electrostatic charge image.

2. The method according to claim 1 in which said evaporating is performed with said surface remote from a development electrode whereby said solid is distributed on the fringes of the charged areas of said electrostatic charge image.

3. The method according to claim 1 in which said evaporating is performed with said surface adjacent to a development electrode whereby said solid is distributed substantially unformly in the charged areas of said electrostatic charge image.

4. The method according to claim 1 in which said solute is a dye whereby said solid forms a visible basrelief image.

5'. The method according to claim 1 in which said solute is an organic polymer whereby said solid forms a has-relief image.

6. The method according to claim 1 in which said solute is one reagent of a color forming reaction and in which a color is formed by reacting said solid with a second reagent of said reaction.

'7. The method according to claim 6 in which the surface on which the electrostatic image is formed contains zinc oxide and in which said solid reacts with the zinc oxide to form dye.

8. The method according to claim 6 in which said sec- 20 ond reagent is applied to said solid forming said hasrelief image after said evaporating.

UNITED ferences Cited STATES PATENTS Fauser et al. 96-1 Gundlach 252-621 Bornarth 252-62.1 Fauser et al 252-62.1 Gundlach et al. 117-93.4 Gunther et al. 11.7-93.4 \Valkup 117-93.4 Pirie 117-37 Olson 117-37 Rickey 117-37 Johnson.

Reithel 117-37 Greig 117-37 Gundlach 117-37 Schmiedel et al 11717.5 Schaifert 117-111 X WILLIAM D. MARTIN, Primary Examiner. G. L. HUBBARD, S. W. ROTHSTEIN, E. CABIC,

Assistani Examiners. 

1. IN A XEROGRAPHIC PROCESS FOR THE PRODUCTION OF A VISIBLE IMAGE CORRESPONDING TO AN IMAGE TO BE RECORDED WHEREIN AN ELECTROSTATIC CHARGE IMAGE CORRESPONDING TO SAID IMAGE TO BE RECORDED IS PRODUCED, ON AN INSULATING SURFACE, THE METHOD OF DEVELOPING SAID ELECTROSTATIC CHARGE IMAGE COMPRISING THE STEPS OF APPLYING TO THE ENTIRE AREA OF SAID SURFACE CONTAINING SAID ELECTROSTATIC CHARGE IMAGE, A LAYER OF AN ELECTRICALLY INSULATING SOLUTION HAVING A SPECIFIC RESISTANCE OF 10**10 OHM-CM. AND A DIELECTRIC CONSTANT LESS THAN 3, WHEREBY SAID ELECTROSTATIC CHARGE IMAGE INDUCES A BAS-RELIEF IMAGE INTO SAID LAYER, 